This invention relates to improvements in techniques of constructing optical holograms in photoresist detector material.
Photoresist material has long been used and written about as a holographic recording medium. The holographic pattern formed from the intersection of two coherent radiation beams is applied to photoresist material and selectively alters the material in accordance with the intensity variation of the holographic interference pattern. Both positive and negative photoresist material can be used for this purpose. After exposure, the photoresist is developed by etching its surface to form a surface relief pattern corresponding to that of the intensity pattern to which the photoresist material was exposed. If a positive photoresist material is utilized, which is the most prevalent, the portions of the detector which are exposed to high intensity radiation are removed faster than those portions which are not. The resulting surface relief hologram reconstructs an image of the object by refracting or reflecting light incident on it into the various order beams.
Currently utilized techniques expose the photoresist material for as short a time as possible with an intensity in the reference beam that is many times the average intensity of the object modified beam across the photoresist detector. It is taught in the literature that the high reference to object modified beam intensity ratio is necessary to minimize distortion by operating on a linear portion of the photoresist characteristic curve. The amount of light energy to which the photoresist material is exposed is minimized by keeping the exposure time short in order to avoid polymerizing the entire surface with the high reference beam intensity bias. After exposure, the photoresist material is developed for a long time in order to obtain a sufficient differential etch.
A basic paper disclosing existing techniques appeared in Applied Optics, Vol. 9, No. 10, Oct. 1970, pp. 2283-2290, by Bartolini et al., entitled "Embossed Hologram Motion Pictures for Television Playback". As described in that paper, a common use of the photoresist material is as a master hologram from which a metal master may be formed for stamping out inexpensive replica holograms by embossing. Another paper directed to the replication steps entitled "Replication of Relief-Phase Holograms for Prerecorded Video" by Bartolini et al. appeared in the Journal of the Electrical Chemical Society, Vol. 120, No. 10, pp. 1408-1413, Oct. 1973.
It is a primary object of the improved photoresist techniques of the present invention to reduce noise and distortion in images reconstructed from photoresist holograms, while maintaining image brightness.